Study On The Mechanism Of Internal And External Factors In The Breakdown Of High Voltage Power Frequency Electric Heating In Oil Shale

The situation of China’s energy supply security is very serious,with oil dependence reaching 71.3% in 2022,and it is urgent to find an alternative energy source.Oil shale,a kind of solid combustible organic sedimentary rock,with high ash content and can obtain shale oil at low-temperature distillation.China has huge geological resources of oil shale and contains shale oil resources of about 47.644 billion tons,which makes oil shale an important potential oil and gas replacement resource in China.Underground in-situ pyrolysis technology is an important way to achieve efficient and clean extraction of oil shale.This method is to heat oil shale in situ to pyrolyze kerogen underground and produce oil and gas,which is then extracted to the surface.The existing in-situ pyrolysis(ICP)technologies for oil shale mainly include conduction heating,convection heating,radiation heating,and chemical heating.A large number of scholars have proposed a variety of ICP methods for oil shale.Among them,in-situ pyrolysis of oil shale by high voltage power frequency electric heating(HVF)has attracted much attention due to its simple process,good economy,and strong adaptability to formation.In this method,two electrodes are buried in the target layer of oil shale,and high-voltage electricity is injected into the formation,which breaks down the oil shale to make its resistivity decrease significantly.Then,frequency power electricity is further connected to heat and pyrolyze the surrounding oil shale by the Joule thermal effect,so that kerogen could be converted into oil and gas.It is evident that the key to the implementation of this method is the process of high voltage breakdown of oil shale.Therefore,in this paper,a series of indoor experimental studies and numerical simulations are carried out to investigate the HVF technology.Based on the thermophysical and electrophysical properties of oil shale,the breakdown process of in-situ oil shale pyrolysis by HVF is analyzed from the perspective of heat generation and heat dissipation,in order to investigate the influence of internal and external factors such as moisture content,oil content,initial temperature and electric field strength on the breakdown,and systematically investigate the mechanism of internal and external factors on oil shale breakdown.The breakdown process was evaluated from the perspectives of breakdown field strength,breakdown time,breakdown type and breakdown energy consumption by conducting oil shale breakdown experiments under different conditions using an HVF test rig,and analyzing and discussing the influence mechanisms under the action of various internal and external factors.The following conclusions were mainly obtained:(1)The physicochemical properties and the release characteristics of products after pyrolysis of oil shale in Huadian were studied by various methods,and quantitative analyses of the electrophysical and thermophysical properties of oil shale at different temperatures,moisture contents and regions were conducted to study the evolution of the electrophysical and thermophysical properties of oil shale under the effect of multiple factors.The results show that the resistivity,dielectric constant,thermal conductivity and specific heat capacity of oil shale show a decreasing trend with the increase of temperature.The moisture content has a significant impact on the electrophysical and thermophysical properties of oil shale.As moisture content increases,the oil shale resistivity decreases exponentially,the dielectric constant decreases,the thermal conductivity increases,and the specific heat capacity increases.The specific heat capacity of oil shale is the result of a combination of many factors,and it is impossible to make an accurate judgment on the relative magnitude of specific heat capacity of oil shale in different regions from the perspective of a single factor only.(2)Indoor experiments on oil shale breakdown were carried out using a selfdesigned high-voltage power-frequency experimental apparatus.The characteristics of samples and products before and after breakdown were analyzed,and the effects of internal and external factors such as moisture content,oil content,electric field strength and initial temperature on the breakdown characteristics of oil shale were discussed and analyzed.The results show that there is a critical electric field strength for the thermal breakdown of oil shale.The critical electric field strength of thermal breakdown of Huadian oil shale is 90-100V/cm;in the range of 100-180V/cm electric field strength,the critical temperature of thermal breakdown of Huadian oil shale is 93-102℃;the greater the electric field strength,the easier the thermal breakdown occurs and the lower the energy consumption.When the electric field strength is greater than 200 V/cm,the electric breakdown occurs in the Huadian oil shale.Increasing the initial temperature increases the difficulty of thermal breakdown of oil shale,which is inconsistent with the general solid thermal breakdown theory.The presence of water is a necessary condition for oil shale to be able to be breakdown.The difficulty of oil shale breakdown gradually increases as the oil content decreases.In the electric field strength range of 100-180 V/cm,the breakdown of Huadian oil shale is a combination of thermal breakdown and electrochemical breakdown.The thermal breakdown of oil shale is the result of superposition of heat generation and heat dissipation.The electric field strength only affects the heat generation process,the initial temperature affects the heat generation and heat dissipation processes,and the moisture content affects both heat generation and heat dissipation processes,and the influence of moisture content is greater than that of initial temperature.The thermal breakdown of oil shale instantly forms breakdown channels,which leads to the fission of the cheese roots inside the channels.(3)A mathematical model of high-voltage power frequency electric heating breakdown based on the assumption of porous medium and electrothermal coupling analysis was established,and the reliability of the model was verified by comparing the temperature distribution of oil shale at different time points of the breakdown process,the temperature change law at the temperature measurement point and the energy consumption.Further,the analysis of the oil shale breakdown process shows that the thermal breakdown process of oil shale is influenced by the combination of electrical conductivity,thermal conductivity and specific heat capacity.The moisture content and oil content affect the electrical conductivity,thermal conductivity and specific heat capacity of oil shale,and their effects on electrical conductivity will promote the rapid penetration of oil shale,but their effects on specific heat capacity will prevent the penetration from occurring.The organic matter such as connected pores,microfractures and caseous roots inside the oil shale are the main control factors leading to its electrochemical breakdown.Residual carbon from oil shale pyrolysis increases the electrical conductivity of oil shale,making it more susceptible to breakdown.(4)According to the process requirements of high-voltage power-frequency electric heating,a 100-ton test scheme for in-situ pyrolyzing by HVF was designed.Firstly,the numerical simulation model was used as a benchmark to simulate the large scale oil shale breakdown process and solve the electrical parameters required for the large scale breakdown test to provide a basis for the field test design.The results show that the critical thermal breakdown electric field strength of the Fuyu oil shale is 62V/cm at an electrode distance of 10 m,and the required voltage is 62,000 V,corresponding to a breakdown time of 31 s.Then,the designed HVF process is a composite subsurface in-situ mining process that uses the formation’s own electrical resistance heat to pyrolyze the oil shale with gas repellent oil and gas.The method introduces the principle of the oil shale’s own electrical resistance heating,and achieves high efficiency and energy saving of the whole extraction process by injecting inert gas to replace oil and gas.The test program includes process concept design,underground engineering design,surface engineering design,and process operation design,which will provide guidance for conducting field tests of HVF technology.The study of this paper can enrich the basic theory of HVF technology,with a view to providing theoretical support and technical guidance for the field test of HVF.